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CHEMICAL REACTIONS AND YIELD
stoichiometry = the numerical relationships between chemical amounts in a reaction.
2C8H18(l) + 25O2 16CO2(g) + 18H2O(g)
From the equation, 16 moles of CO2 (a greenhouse gas resulting in global warming) are produced for every 2 moles of octane burned (by combustion).
Estimate the mass of CO2 produced in 2004 by the combustion of 3.4 x 1015 g gasoline.
Solution: number of moles = mass/molar mass; molar mass = molecular mass
molecular masses: C8H18 = 114.22; CO2 = 44.01 gmol-1
4.4 x 1015/114.22 = 3.882 x 1013 moles C8H18.
stoichiometric ratio CO2: C8H18 = 16:2 3.882 x 1013 x (16/2) = 3.056 x 1014 moles CO2
= 1.3 x 1016 g CO2.
limiting reactant (or reagent) = the reactant that limits the amount of product
excess reactants = reactants not completely consumed
theoretical yield = the amount of product that can be made from the limiting reactant
actual yield = the amount of product that is made in a reaction actual yield
generally less than the theoretical yield, never more!
percent yield = the efficiency of product recovery
When 28.6 kg of C reacts with 88.2 kg of TiO2, 42.8 kg of Ti are obtained. Find the Limiting Reactant, Theoretical Yield, and Percent Yield.
TiO2(s) + 2 C(s) ® Ti(s) + 2CO(g)
Solution: 1 mole TiO2 gives 1 mole Ti; 2 moles C gives 1 mole Ti.
molar masses C = 12.01; TiO2 = 79.87; Ti = 47.87 gmol-1.
Number of moles of C = 2.38 x 103 moles 1.191 x 103 moles Ti
Number of moles of TiO2 = 2.38 x 103 moles 1.104 x 103 moles Ti
TiO2 = limiting reactant 1.104 x 103 x 47.87 = 52.9 kg Ti = theoretical yield
Percent yield = 100% x 42.8 / 52.9 = 80.9 %.
PRACTICE EXAMPLE ONE
Mining companies use the following reaction to obtain iron from iron ore:
Fe2O3(s) + 3CO(g) 2Fe(s) + 3CO2(g)
The reaction of 167g of Fe2O3 with 85.8g of CO produces 72.3g Fe. Find the limiting reagent, theoretical yield and percent yield.
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Making aqueous solutions
When an ionic compound dissolves in water, the water molecules surround the cations and anions; e.g. KCl(s) K+(aq) + Cl-(aq). Heating the solution increases solubility. Substances such as potassium chloride or hydrogen chloride (strong acid) that completely dissociate into ions are strong electrolytes (solution conducts electrical current). Weak electrolytes dissolve mostly as molecules but partly dissociate into ions:
dilute (淡的)solutions = small amount of solute (溶质)compared to solvent (溶剂)
concentrated (浓)solutions = large amount of solute compared to solvent
Find the molarity, M (or concentration, C) of a solution that has 25.5 g KBr dissolved in 1.75 L of solution
Solution: molar mass KBr = 119.00 gmol-1
number of moles = 25.5 / 119 = 0.214 mol KBr
molarity = 0.214 / 1.75 = 0.122 M (or mol dm-3).
Dilution = make a solution less concentrated
moles solute in solution 1 = moles solute in solution 2
M1V1 = M2V2
To what volume should you dilute 0.200 L of 15.0 M NaOH to make 3.00 M NaOH?
Solution: M2 = M1V1 / V2 = 15 x 0.2 / 3 = 1.0 M
PRACTICE EXAMPLE TWO
What volume of a 6.00 M NaNO3 solution should you use to make 0.525 L of a 1.20 M NaNO3 solution?
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What volume of 0.150 M KCl is required to completely react with 0.150 L of 0.175 M Pb(NO3)2 in the reaction 2KCl(aq) + Pb(NO3)2(aq) ® PbCl2(s) + 2 KNO3(aq)?
1 L Pb(NO3)2 = 0.175 mol, 1 L KCl = 0.150 mol,
stoichiometry: 1 mol Pb(NO3)2 reacts with 2 mol KCl
Number of moles of Pb(NO3)2 used = molarity x volume = 0.0263 moles
Number of moles of KCl used = 0.02625 moles x 2 = 0.0525 moles
volume KCl = moles / molarity = 0.0525 /0.150 = 0.35 L
PRACTICE EXAMPLE THREE
Consider the following reaction:
Li2S(aq) + Co(NO3)2(aq) ® 2LiNO3(aq) + CoS(aq)
What volume of 0.150 M Li2S solution is required to completely react with 125 mL of 0.250 M Co(NO3)2?
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soluble (可溶性) = compounds that dissolve in a solvent (e.g. NaCl dissolves in water)
insoluble (不溶性) = compounds that do not dissolve (e.g. AgCl does not)
Most Group I compounds are soluble in water (e.g. NaOH, Na2SO4). Some Group II compounds are soluble (e.g. CaCl2), slightly soluble e.g. Ca(OH)2, others are not (e.g. CaSO4). Many ammonium NH4+ compounds are soluble (e.g. NH4Cl) and ALL NITRATES (e.g. Ca(NO3)2 ) are soluble. See book table 4.1, p.149.
Precipitation (沉淀)reactions = reactions between aqueous solutions of ionic compounds that produce an ionic compound that is insoluble (precipitate) in water:
2KI(aq) + Pb(NO3)2(aq) ® PbI2(s) + 2 KNO3(aq); PbI2(s) is the precipitate
Writing and simplifying equations
molecular equations = equations which describe the chemicals put into the water and the product molecules molecular equations, e.g.
2KI(aq) + Pb(NO3)2(aq) ® PbI2(s) + 2 KNO3(aq)
complete ionic equations = equations which describe the actual dissolved species:
2K+(aq) + 2I-(aq) + Pb2+(aq) + 2NO32-(aq) ® 2K+(aq) + 2NO32-(aq) + PbI2(s)
• ions that are both reactants and products are called spectator ions (they do not react and remain in the solution)
net ionic equation = an ionic equation in which the spectator ions are removed:
Pb2+(aq) + 2I-(aq) ® PbI2(s)
neutralization reactions = the acid and base neutralize each other’s properties
2HNO3(aq) + Ca(OH)2(aq) ® Ca(NO3)2(aq) + 2 H2O(l)
the net ionic equation for an acid-base reaction is:
H+(aq) + OH-(aq) ® H2O(l)
PRACTICE EXAMPLE FOUR
Write complete ionic and net ionic equations for each reaction:
K2SO4(aq) + CaI2(aq) ® CaSO4(s) + KI(aq)
NH4Cl(aq) + NaOH(aq) ® NH3(g) + NaCl(aq) + H2O(l)
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acids ionize in water to form H+ ions
more precisely, the H from the acid molecule is donated to a water molecule to form hydronium ion, H3O+. A proton (H+) cannot exist on its own in water!
bases dissociate in water to form OH- ions
bases, such as NH3, that do not contain OH- ions, produce OH- by pulling H off water molecules
acid + base ® salt + water
titration - a solution’s concentration is determined by reacting it with another material and using stoichiometry (involving a chemical equation and calculation). An indicator permanently changes colour at the end-point (just finished).
For example, if we know the concentration and precise volume of the alkali but only the volume of acid of unknown concentration, we can determine its concentration. Titration can be used for many ionic reactions.
n1A + n2B ® products (e.g. salt + water)
12.54 mL of 0.100 M NaOH(aq) reacts 10.00 mL of HCl(aq) solution. What is the concentration of the acid? The formula can be used with mL (for both solutions) or converted to L for both solutions.
NaOH(aq) + HCl(aq) ® NaCl(aq) + H2O(l)
C1 concentration = 0.100 M NaOH(aq); C2 = ?
V1 volume = 12.54 mL NaOH(aq); V2 = 10.00 mL
n1 stoichiometric value = 1 for NaOH; n2 = 1 for HCl
C2 = 0.100 x 12.54 x 1 / 1 x 10.00 = 0.125 M HCl(aq)
Some neutralization reactions evolve gases; the intermediate carbonic acid (H2CO3) thus decomposes, e.g.
Na2CO3(aq) + 2HNO3(aq) ® 2NaNO3(aq) + CO2(g) + H2O(l)
PRACTICE EXAMPLE FIVE
A 30.00 mL sample of an unknown H3PO4 solution is titrated with a 0.100 M NaOH solution. The end-point (equivalence point) is reached when 26.38 mL of NaOH solution is added. What is the concentration of the unknown H3PO4 solution?
3NaOH(aq) + H3PO4(aq) ® Na3PO4(aq) + 3H2O(l)
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REDOX REACTIONS
REDOX (oxidation-reduction) reactions = transferring electrons from one atom to another
Many involve the use of O2, Cl2 etc.
4 Fe(s) + 3O2(g) ® 2 Fe2O3(s); the iron has transferred its electrons to the oxygen atoms
Combustion (molecular - involve the use of O2) is also REDOX, e.g.
H2(g) + O2(g) ® 2H2O(l)
oxidation occurs when an atom’s oxidation state increases during a reaction (Fe(0) to Fe +3). LOSS OF ELECTRONS
reduction occurs when an atom’s oxidation state decreases during a reaction (O (0) to O (-2). GAIN OF ELECTRONS
In a REDOX reaction, the oxidizing agent is always reduced and the reducing agent always oxidized.
C +2S ® CS2 carbon is the reducing agent (0 to +4) and sulphur the oxidizing agent
(0 to -2).
Oxidation state and ionic charges are not to be confused; for single atoms with a charge (e.g. Fe3+, O2-) the oxidation state is often the charge!
· oxygen is nearly always -2 (exception KO2).
· Group I = +1; Group 2 = +2
· Transition metals have variable oxidation states (e.g. chromium from +2 to +6)
K2Cr2O7 : Cr = +6 = potassium chromate(VI)
· elements (e.g. O2 oxidation state = zero (0).
· Metals have positive oxidation states.
· In a neutral atom or formula the sum (S) of the oxidation states = 0.
· An ion is equal to the charge of the ion.
SO42-, and H2SO4 sulphur has oxidation state of +6.
PRACTICE EXAMPLE SIX
What is the oxidation state of Cl in each ion?
1. a) ClO- b)ClO2- c)ClO3- d)ClO4-
2. Determine the oxidizing and reducing agent:
a) Al(s) + 3Ag+(aq) ® Al3+(aq) + 2Ag(s)
b) NaI2(aq) + Cl2(g) ® 2NaCl(aq) + I2(aq)
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COMBUSTION REACTIONS
We have encountered several combustion reactions so far. Common ones include burning elemental carbon, hydrogen, sulphur, phosphorous, hydrocarbons (e.g. methane) or alcohol in air or oxygen. They give off heat (exothermic, DH < 0 typically about 2000 KJmol-1) and light. Equations must be balanced in an exam (number of atoms on the left = number of atoms on the right).